Method for transmitting and receiving data in wireless communication system and apparatus for the same

ABSTRACT

A method for transmitting and receiving data in a wireless communication system and an apparatus for the method are disclosed. More specifically, The present invention provides a method for transmitting downlink data in a wireless communication system can comprise mapping, by a eNB, first downlink data into a physical downlink shared channel (PDSCH) region according to a radio frame structure based on a first transmission time interval (TTI), mapping, by the eNB, second downlink data to a short PDSCH (sPDSCH) region according to a radio frame structure based on a second TTI, and transmitting, by the eNB, the first and the second downlink data.

TECHNICAL FIELD

The present invention relates to a wireless communication system andmore specifically, a method for transmitting and receiving dataemploying a radio frame structure based on a 2-level transmission timeinterval (TTI) in a wireless communication system and an apparatussupporting the method.

BACKGROUND ART

Mobile communication systems have been developed to provide voiceservices, while guaranteeing user activity. Service coverage of mobilecommunication systems, however, has extended even to data services, aswell as voice services, and currently, an explosive increase in traffichas resulted in shortage of resource and user demand for a high speedservices, requiring advanced mobile communication systems.

The requirements of the next-generation mobile communication system mayinclude supporting huge data traffic, a remarkable increase in thetransfer rate of each user, the accommodation of a significantlyincreased number of connection devices, very low end-to-end latency, andhigh energy efficiency. To this end, various techniques, such as smallcell enhancement, dual connectivity, massive Multiple Input MultipleOutput (MIMO), in-band full duplex, non-orthogonal multiple access(NOMA), supporting super-wide band, and device networking, have beenresearched.

DISCLOSURE Technical Problem

The present invention provides a radio frame structure based on a2-level TTI for a user equipment (UE) requiring low latency datacommunication in the next generation broadband wireless communicationsystem.

Also, the present invention provides a short TTI frame structureintended for low latency transmission in sub-bands within the same bandor in a particular band to minimize an effect on legacy UEs.

Also, the present invention provides a method for transmitting andreceiving data employing a radio frame structure based on a newlydefined 2-level TTI.

The technical problems solved by the present invention are not limitedto the above technical problems and those skilled in the art mayunderstand other technical problems from the following description.

Technical Solution

In one aspect of the present invention, a method for transmittingdownlink data in a wireless communication system comprises mapping, by aeNB, first downlink data into a physical downlink shared channel (PDSCH)region according to a radio frame structure based on a firsttransmission time interval (TTI); mapping, by the eNB, second downlinkdata to a short PDSCH (sPDSCH) region according to a radio framestructure based on a second TTI; and transmitting, by the eNB, the firstand the second downlink data.

In another aspect of the present invention, an eNB transmitting downlinkdata in a wireless communication system comprises a radio frequency (RF)unit for transmitting and receiving a radio signal and a processor,wherein the processor is configured to map first downlink data into aphysical downlink shared channel (PDSCH) region according to a radioframe structure based on a first TTI, to map second downlink data intoan sPDSCH region according to a radio frame structure based on a secondTTI, and to transmit the first and the second downlink data.

In a yet another aspect of the present invention, a method for receivingdownlink data in a wireless communication system comprises receiving, byan User Equipment (UE), first downlink data in a PDSCH region accordingto a radio frame structure based on a first TTI and receiving, by theUE, second downlink data in an sPDSCH region according to a radio framestructure based on a second TTI.

In a still another aspect of the present invention, a UE receivingdownlink data in a wireless communication system comprises an RF unitfor transmitting and receiving a radio signal and a processor, whereinthe processor is configured to receive first downlink data in a PDSCHregion according to a radio frame structure based on a first TTI and toreceive second downlink data in an sPDSCH region according to a radioframe structure based on a second TTI.

Preferably, a downlink cell into which the first downlink data aremapped is the same as a downlink cell into which the second downlinkdata are mapped, and one or more sub-bands within the downlink cell canbe formed according to a radio frame structure based on the second TTI.

Preferably, the first downlink cell into which a first downlink data aremapped can be different from the second downlink cell into which asecond downlink data are mapped, and the first downlink cell and thesecond downlink cell can be aggregated.

Preferably, one or more sub-bands within the second downlink cell can beformed according to a radio frame structure based on the second TTI.

Preferably, size of the second TTI can be the same as the number ofsymbols of the sPDSCH region.

Preferably, size of the second TTI can be the same as the sum of thenumber of symbols of a short physical downlink control channel (sPDCCH)region and the number of symbols of the sPDSCH region.

Preferably, the first downlink cell can be configured as a secondarycell, and the second downlink cell can be configured as a primary cell.

Preferably, both of the first downlink cell and the second downlink cellcan be configured as primary cells.

Preferably, transmitting, by the eNB, information about a radio framestructure based on the second TTI through a radio resource control (RRC)message can be further included.

Preferably, the RRC message can correspond to one of a systeminformation message, an RRC connection setup message, an RRC connectionreconfiguration message, or an RRC connection reestablishment message.

Advantageous Effects

By transmitting and receiving data in a radio frame structure based on a2-level TTI, the present invention can reduce roundtrip OTA latency fromthe time of data transmission until a response signal with respect tothe corresponding data is received.

Also, the present invention can minimize the effect on legacy UEs as ashort TTI frame structure intended for low latency transmission insub-bands within the same band or in a particular band is employed.

The effects of the present invention are not limited to theabove-described effects and other effects which are not described hereinwill become apparent to those skilled in the art from the followingdescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention.

FIG. 1 illustrates physical channels and a view showing physicalchannels used for in the 3GPP LTE/LTE-A system to which the presentinvention can be applied.

FIG. 2 illustrates a radio frame structure in a wireless communicationsystem to which the present invention can be applied.

FIG. 3 shows an example of a resource grid for one downlink slot in thewireless communication system to which the present invention can beapplied.

FIG. 4 shows a structure of a downlink subframe in the wirelesscommunication system to which the present invention can be applied.

FIG. 5 shows a structure of an uplink subframe in the wirelesscommunication system to which the present invention can be applied.

FIG. 6 shows a radio frame structure for transmission of asynchronization signal (SS) in a wireless communication system to whichthe present invention can be applied.

FIG. 7 illustrates a radio frame structure for transmitting asynchronization signal (SS) in a wireless communication system to whichthe present invention can be applied.

FIG. 8 represents an example of component carrier and carrieraggregation in the wireless communication system to which the presentinvention can be applied.

FIG. 9 illustrates one example of carrier aggregation in a wirelesscommunication system to which the present invention can be applied.

FIG. 10 illustrates one example of a subframe structure according tocross carrier scheduling in a wireless communication system to which thepresent invention can be applied.

FIG. 11 illustrates a delay in wireless transmission and reception inthe 3GPP LTE/LTE-A system to which the present invention can be applied.

FIG. 12 illustrates a radio frame structure according to one embodimentof the present invention.

FIG. 13 illustrates a radio frame structure according to one embodimentof the present invention.

FIG. 14 illustrates a radio frame structure according to one embodimentof the present invention.

FIG. 15 illustrates a radio frame structure according to one embodimentof the present invention.

FIG. 16 illustrates a radio frame structure according to one embodimentof the present invention.

FIG. 17 illustrates a radio frame structure according to one embodimentof the present invention.

FIG. 18 illustrates a radio frame structure according to one embodimentof the present invention.

FIG. 19 illustrates a method for transmitting information about a shortTTI frame structure according to one embodiment of the presentinvention.

FIG. 20 illustrates a method for configuring carrier aggregation for acarrier-wise 2-level frame structure according to one embodiment of thepresent invention.

FIG. 21 illustrates a method for configuring carrier aggregation forcarrier-wise 2-level frame structure.

FIG. 22 illustrates a method for transmitting and receiving dataaccording to one embodiment of the present invention.

FIG. 23 illustrates a method for transmitting downlink data according toone embodiment of the present invention.

FIG. 24 illustrates a delay in radio transmission and receptionemploying a short TTI radio frame structure according to one embodimentof the present invention.

FIG. 25 is a block diagram of a wireless communication apparatusaccording to an embodiment of the present invention.

BEST MODE

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The detailed description set forth below in connection withthe appended drawings is a description of exemplary embodiments and isnot intended to represent the only embodiments through which theconcepts explained in these embodiments can be practiced. The detaileddescription includes details for the purpose of providing anunderstanding of the present invention. However, it will be apparent tothose skilled in the art that these teachings may be implemented andpracticed without these specific details.

In some instances, known structures and devices are omitted, or areshown in block diagram form focusing on important features of thestructures and devices, so as not to obscure the concept of the presentinvention.

In the embodiments of the present invention, the enhanced Node B (eNodeB or eNB) may be a terminal node of a network, which directlycommunicates with the terminal. In some cases, a specific operationdescribed as performed by the eNB may be performed by an upper node ofthe eNB. Namely, it is apparent that, in a network comprised of aplurality of network nodes including an eNB, various operationsperformed for communication with a terminal may be performed by the eNB,or network nodes other than the eNB. The term ‘eNB’ may be replaced withthe term ‘fixed station’, ‘base station (BS)’, ‘Node B’, ‘basetransceiver system (BTS),’, ‘access point (AP)’, etc. The term ‘userequipment (UE)’ may be replaced with the term ‘terminal’, ‘mobilestation (MS)’, ‘user terminal (UT)’, ‘mobile subscriber station (MSS)’,‘subscriber station (SS)’, ‘Advanced Mobile Station (AMS)’, ‘Wirelessterminal (WT)’, ‘Machine-Type Communication (MTC) device’,‘Machine-to-Machine (M2M) device’, ‘Device-to-Device (D2D) device’,wireless device, etc.

In the embodiments of the present invention, “downlink (DL)” refers tocommunication from the eNB to the UE, and “uplink (UL)” refers tocommunication from the UE to the eNB. In the downlink, transmitter maybe a part of eNB, and receiver may be part of UE. In the uplink,transmitter may be a part of UE, and receiver may be part of eNB.

Specific terms used for the embodiments of the present invention areprovided to aid in understanding of the present invention. Thesespecific terms may be replaced with other terms within the scope andspirit of the present invention.

The embodiments of the present invention can be supported by standarddocuments disclosed for at least one of wireless access systems,Institute of Electrical and Electronics Engineers (IEEE) 802, 3rdGeneration Partnership Project (3GPP), 3GPP Long Term Evolution (3GPPLTE), LTE-Advanced (LTE-A), and 3GPP2. Steps or parts that are notdescribed to clarify the technical features of the present invention canbe supported by those documents. Further, all terms as set forth hereincan be explained by the standard documents.

Techniques described herein can be used in various wireless accesssystems such as Code Division Multiple Access (CDMA), Frequency DivisionMultiple Access (FDMA), Time Division Multiple Access (TDMA), OrthogonalFrequency Division Multiple Access (OFDMA), Single Carrier-FrequencyDivision Multiple Access (SC-FDMA), ‘non-orthogonal multiple access(NOMA)’, etc. CDMA may be implemented as a radio technology such asUniversal Terrestrial Radio Access (UTRA) or CDMA2000. TDMA may beimplemented as a radio technology such as Global System for Mobilecommunications (GSM)/General Packet Radio Service (GPRS)/Enhanced DataRates for GSM Evolution (EDGE). OFDMA may be implemented as a radiotechnology such as IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE802.20, Evolved-UTRA (E-UTRA) etc. UTRA is a part of Universal MobileTelecommunication System (UMTS). 3GPP LTE is a part of Evolved UMTS(E-UMTS) using E-UTRA. 3GPP LTE employs OFDMA for downlink and SC-FDMAfor uplink. LTE-A is an evolution of 3GPP LTE.

For clarity, this application focuses on the 3GPP LTE/LTE-A system.However, the technical features of the present invention are not limitedthereto.

General System to which the Present Invention May be Applied

FIG. 1 illustrates physical channels and a view showing physicalchannels used for in the 3GPP LTE/LTE-A system to which the presentinvention can be applied.

When a UE is powered on or when the UE newly enters a cell, the UEperforms an initial cell search operation such as synchronization with aBS in step S101. For the initial cell search operation, the UE mayreceive a Primary Synchronization Channel (P-SCH) and a SecondarySynchronization Channel (S-SCH) from the BS so as to performsynchronization with the BS, and acquire information such as a cell ID.

Thereafter, the UE may receive a physical broadcast channel (PBCH) fromthe BS and acquire broadcast information in the cell. Meanwhile, the UEmay receive a Downlink Reference signal (DL RS) in the initial cellsearch step and confirm a downlink channel state.

The UE which completes the initial cell search may receive a PhysicalDownlink Control Channel (PDCCH) and a Physical Downlink Shared Channel(PDSCH) corresponding to the PDCCH, and acquire more detailed systeminformation in step S102.

Thereafter, the UE may perform a random access procedure in steps S303to S306, in order to complete the access to the BS. For the randomaccess procedure, the UE may transmit a preamble via a Physical RandomAccess Channel (PRACH) (S103), and may receive a message in response tothe preamble via the PDCCH and the PDSCH corresponding thereto (S104).In contention-based random access, a contention resolution procedureincluding the transmission of an additional PRACH (S105) and thereception of the PDCCH and the PDSCH corresponding thereto (S106) may beperformed.

The UE which performs the above-described procedure may then receive thePDCCH/PDSCH (S107) and transmit a Physical Uplink Shared Channel(PUSCH)/Physical Uplink Control Channel (PUCCH) (S108), as a generaluplink/downlink signal transmission procedure.

Control information transmitted from the UE to the BS is collectivelyreferred to as uplink control information (UCI). The UCI includes hybridautomatic repeat and request acknowledgement/negative-acknowledgement(HARQ ACK/NACK), scheduling request (SR), channel quality information(CQI), precoding matrix indicator (PMI), rank indication (RI), etc. Inthe embodiments of the present invention, CQI and/or PMI are alsoreferred to as channel quality control information.

In general, although a UCI is periodically transmitted via a PUCCH inthe LTE system, this may be transmitted through a PUSCH if controlinformation and traffic data are simultaneously transmitted. Inaddition, a UCI may be aperiodically transmitted via a PUSCH accordingto a network request/instruction.

FIG. 2 illustrates a radio frame structure in a wireless communicationsystem to which the present invention can be applied.

A method for distinguishing radio resources used for downlinktransmission from the resources for uplink transmission is called‘duplex’.

Frequency division duplex (FDD) denotes two-way communication whereseparate frequency bands are used for downlink and uplink transmission.According to the FDD scheme, uplink transmission and downlinktransmission are carried out in the respective frequency bands.

Time division duplex (TDD) denotes duplex communication links whereuplink is separated from downlink by allocating different time durationsin the same frequency band.

According to the TDD scheme, uplink transmission and downlinktransmission occupy the same frequency band but are carried out indifferent time intervals. Channel responses in the TDD scheme areactually reciprocal. The reciprocity indicates that a downlink channelresponse is almost the same as a uplink channel response in a givenfrequency band. Therefore, a wireless communication system based on theTDD scheme provides such an advantage that a downlink channel responsecan be obtained from a uplink channel response. Since uplinktransmission and downlink transmission according to the TDD scheme arecarried out in different time slots across the whole frequency band,downlink transmission by an eNB and uplink transmission by a UE cannotbe carried out simultaneously. In a TDD system where uplink transmissionand downlink transmission are carried out in units of a subframe, uplinktransmission and downlink transmission are carried out in differentsubframes from each other.

The 3GPP LTE/LTE-A standard specifies a radio frame structure type 1which can be applied to the FDD scheme and a type 2 radio framestructure which can be applied to the TDD scheme.

FIG. 2(a) illustrates a radio frame structure type 1. A radio frameconsists of 10 subframes. One subframe comprises two slots in the timedomain. Transmission time interval (TTI) refers to the duration fortransmission of one subframe. For example, length of one subframe can be1 ms, while length of one slot can be 0.5 ms.

One slot comprises a plurality of orthogonal frequency divisionmultiplexing (OFDM) symbols in the time domain and a plurality ofresource blocks (RBs) in the frequency domain. Since the 3GPP LTEstandard utilizes OFDMA for downlink transmission, the OFDM symbol isintended to express one symbol period. One OFDM symbol can refer to oneSC-FDMA symbol or one symbol period. A resource block is a unit forresource allocation and comprises a plurality of contiguous subcarrierswithin one slot.

FIG. 2(b) illustrates a frame structure type 2. A radio frame type 2consists of two half frames and each half frame consists of 5 subframes,a downlink pilot time slot (DwPTS), a guard period (GP), and a uplinkpilot time slot (UpPTS) of which one subframe comprises two slots. TheDwPTS is used for initial cell search, synchronization, or channelestimation in a UE. The UpPTS is used for channel estimation andsynchronization of uplink transmission with a UE. The GP is intended toremove interference exerted on uplink transmission due to a multi-pathdelay of a downlink signal between uplink and downlink transmission.

In the type 2 frame structure of the TDD system, uplink-downlinkconfiguration represents a rule as to whether uplink and downlink areallocated (or reserved) with respect to all of the subframes. Table 1illustrates uplink-downlink configuration.

TABLE 1 Uplink- Downlink- Downlink to-Uplink configu- Switch-pointSubframe number ration periodicity 0 1 2 3 4 5 6 7 8 9 0 5 ms D S U U UD S U U U 1 5 ms D S U U D D S U U D 2 5 ms D S U D D D S U D D 3 10 msD S U U U D D D D D 4 10 ms D S U U D D D D D D 5 10 ms D S U D D D D DD D 6 5 ms D S U U U D S U U D

With respect to Table 1, for each subframe of a radio frame, ‘D’represents a subframes for downlink transmission, ‘U’ represents asubframe for uplink transmission′, and ‘S’ represents a special subframeconsisting of three-types of, fields: DwPTS, GP, and UpPTS. There can beseven different uplink-downlink configurations, where positions and/orthe number of downlink subframes, special subframes, and uplinksubframes of each configuration are different from those of the others.

A time point at which downlink transmission is changed to uplinktransmission or vice versa is called a switching point. The switch-pointperiodicity refers to a period at which switching between a uplinksubframe and a downlink subframe is repeated in the same manner, and aswitch-point periodicity of 5 ms or 10 ms is supported. In the case of aswitch-point periodicity of 5 ms, a special subframe(s) is defined forevery half-subframe, while, in the case of a switch-point periodicity of10 ms, the special subframes(s) are defined only for a first half-frame.

For each configuration, the 0-th, 5-th subframe, and DwPTS correspond totime slots intended only for downlink transmission. UpPTS and a specialsubframe right next to the subframe are always used for uplinktransmission.

The uplink-downlink configuration described above can be known to bothof the eNB and the UE as system information. By transmitting only theindex of a uplink-downlink configuration each time the configuration ischanged, the eNB can inform the UE of the change in the uplink-downlinkallocation state of a radio frame. Also, configuration information canbe transmitted as downlink control information through a physicaldownlink control channel (PDCCH) in the same manner as other schedulinginformation, and may be transmitted as broadcast information commonly toall of the UEs within a cell through a broadcast channel.

The radio frame structure is only an example, and the number ofsub-carriers included in a radio frame or the number of slots includedin a sub-frame, and the number of OFDM slots included in a slot can bechanged in various ways.

FIG. 3 shows an example of a resource grid for one downlink slot in thewireless communication system to which the present invention can beapplied.

Referring to the FIG. 3, the downlink slot includes a plurality of OFDMsymbols in a time domain. It is described herein that one downlink slotincludes 7 OFDMA symbols and one resource block includes 12 subcarriersfor exemplary purposes only, and the present invention is not limitedthereto.

Each element on the resource grid is referred to as a resource element,and one resource block includes 12×7 resource elements. The resourceelement on the resource grid may be identified by an index pair (k, l)in the slot. Here, k (k=0, . . . , NRBx12-1) denotes an index ofsubcarrier in the frequency domain, and l (l=0, . . . , 6) denotes anindex of symbol in the time domain. The number NDL of resource blocksincluded in the downlink slot depends on a downlink transmissionbandwidth determined in a cell.

FIG. 4 shows a structure of a downlink subframe in the wirelesscommunication system to which the present invention can be applied.

Referring to the FIG. 4, a maximum of three OFDM symbols located in afront portion of a first slot in a subframe correspond to a controlregion to be assigned with control channels. The remaining OFDM symbolscorrespond to a data region to be assigned with physical downlink sharedchannels (PDSCHs).

Examples of downlink control channels used in the 3GPP LTE include aphysical control format indicator channel (PCFICH), a physical downlinkcontrol channel (PDCCH), a physical hybrid-ARQ indicator channel(PHICH), etc. The PCFICH transmitted in a 1st OFDM symbol of a subframecarries information regarding the number of OFDM symbols (i.e., a sizeof a control region) used for transmission of control channels in thesubframe. Control information transmitted over the PDCCH is referred toas downlink control information (DCI). The DCI transmits uplink resourceassignment information, downlink resource assignment information, anuplink transmit power control (TPC) command for any UE groups, etc. ThePHICH carries an acknowledgement (ACK)/not-acknowledgement (NACK) signalfor an uplink hybrid automatic repeat request (HARQ). That is, theACK/NACK signal for uplink data transmitted by a UE is transmitted overthe PHICH.

A BS determines a PDCCH format according to DCI to be transmitted to aUE, and attaches a cyclic redundancy check (CRC) to control information.The CRC is masked with a unique identifier (referred to as a radionetwork temporary identifier (RNTI)) according to an owner or usage ofthe PDCCH. If the PDCCH is for a specific UE, a unique identifier (e.g.,cell-RNTI (C-RNTI)) of the UE may be masked to the CRC. Alternatively,if the PDCCH is for a paging message, a paging indication identifier(e.g., paging-RNTI (P-RNTI)) may be masked to the CRC. If the PDCCH isfor system information, a system information identifier (e.g., systeminformation-RNTI (SI-RNTI)) may be masked to the CRC. To indicate arandom access response that is a response for transmission of a randomaccess preamble of the UE, a random access-RNTI (RA-RNTI) may be maskedto the CRC.

FIG. 5 shows a structure of an uplink subframe in the wirelesscommunication system to which the present invention can be applied.

Referring to the FIG. 5, the uplink subframe can be divided in afrequency domain into a control region and a data region. The controlregion is allocated with a physical uplink control channel (PUCCH) forcarrying uplink control information. The data region is allocated with aphysical uplink shared channel (PUSCH) for carrying user data. In caseof being indicated from higher layer, UE can simultaneously transmit thePUCCH and the PUSCH.

The PUCCH for one UE is allocated to an RB pair in a subframe. RBsbelonging to the RB pair occupy different subcarriers in respective twoslots. This is called that the RB pair allocated to the PUCCH isfrequency-hopped in a slot boundary.

FIG. 6 shows a radio frame structure for transmission of asynchronization signal (SS) in a wireless communication system to whichthe present invention can be applied.

In particular, FIG. 6 illustrates a radio frame structure fortransmission of a synchronization signal and a PBCH in frequencydivision duplex (FDD) mode, where FIG. 6(a) illustrates a transmissionposition of an SS and a PBCH in a radio frame employing normal cyclicprefixes (CPs), and FIG. 6(b) illustrates a transmission position of anSS and a PBCH in a radio frame employing extended CP.

In case a UE is powered up or enters a new cell, the UE carries out aninitial cell search procedure to obtain time and frequencysynchronization with the cell and to detect a physical cell identity ofthe cell. To this purpose, the UE can receive from the eNB asynchronization signal, for example, a primary synchronization signal(PSS) and a secondary synchronization signal (SSS) to synchronize withthe eNB and obtain information about a cell identity (ID).

In the following, an SS will be described in more detail with referenceto FIG. 6.

An SS is further divided into a PSS and an SSS. A PSS is used to obtaintime domain synchronization and/or frequency domain synchronization suchas OFDM symbol synchronization and slot synchronization, and an SSS isused to obtain frame synchronization and cell group ID and/or CPconfiguration of a cell (namely, usage information of normal CP orextended CP).

With reference to FIG. 6, a PSS and an SSS are transmitted respectivelyfrom two OFDM symbols of each radio frame in the time domain. To bespecific, an SS is transmitted from the first slot of subframe 0 and thefirst slot of subframe 5 by taking account of the length of a GSM(Global System for Mobile communication) frame, 4.6 ms, to facilitatemeasurement of inter radio access technology (inter-RAT). In particular,a PSS is transmitted respectively from the last OFDM symbol of the firstslot of subframe 0 and, the last OFDM symbol of the first slot ofsubframe 5; and an SSS is transmitted respectively from the next to lastOFDM symbol of the first slot of subframe 0 and the next to last OFDMsymbol of the first slot of subframe 5.

Boundaries of the corresponding radio frames can be detected through theSSS. A PSS is transmitted from the very last OFDM symbol of thecorresponding slot, and an SSS is transmitted from the OFDM symbol rightbefore the PSS. A transmission diversity scheme for the SS makes use ofa single antenna port only and is not separately defined in thestandard. In other words, single antenna port transmission ortransparent transmission to a UE (for example, precoding vectorswitching (PVS), time switched diversity (TSTD), cyclic delay diversity(CDD)) can be used for transmission diversity of an SS.

With reference to FIG. 6, since a PSS is transmitted every 5 ms, bydetecting the PSS, the UE can know that the corresponding subframe iseither of subframe 0 or subframe 5, but is unable to which one of thetwo the corresponding subframe corresponds. Therefore, with the PSSalone, the UE cannot recognize radio frame boundaries. In other words,frame synchronization cannot be attained from the PSS only. The UEdetects radio frame boundaries by detecting the SSS which is transmittedtwice within one radio frame but transmitted in a different sequence.

FIG. 7 illustrates a radio frame structure for transmitting asynchronization signal (SS) in a wireless communication system to whichthe present invention can be applied.

With reference to FIG. 7, a PSS and an SSS are mapped to 6 RBs locatedin the center of downlink system bandwidth. The total number of RBs indownlink transmission can vary depending upon system bandwidth (forexample, 6 RBs to 110 RBs). Since the PSS and the SSS are mapped to 6RBs located in the center of the downlink system bandwidth, the UE candetect the PSS and the SSS by using the same method independently of thedownlink system bandwidth.

The PSS and the SSS are all composed of a sequence of length 62.Therefore, among the 6 RBs, the PSS and the SSS are mapped to thecentral 62 subcarriers located at both sides of a DC subcarrier, and 5subcarriers located respectively at the both ends of the 6RBs and the DCsubcarrier are not used.

A UE can obtain a physical layer cell ID by using a particular sequenceof the PSS and the SSS. In other words, an SS can represent a total of504 unique physical layer cell IDs through a combination of 3 PSSs and168 SSs.

Put differently, the physical layer cell IDs are grouped to 168 physicallayer cell ID groups, each of which includes three unique IDs, so thateach physical layer cell ID can belong to only one physical layer cellID group. Therefore, a physical layer cell, ID Ncell ID=3N(1) ID+N(2)ID, is uniquely defined by the number N(1) ID ranging from 0 to 167 andrepresenting a physical layer cell ID group; and the number N(2) IDranging from 0 to 2 and representing the physical layer ID within thephysical layer cell ID group.

The UE, by detecting the PSS, can distinguish one of the three uniquephysical layer IDs and can identify one of the 168 physical layer cellIDs associated with the physical layer ID by detecting the SSS.

The PSS is generated based on a Zadoff-Chu (ZC) sequence. Three ZC PSSscorresponding respectively to the three physical layer IDs within eachphysical layer cell ID group are used.

The SSS is generated based on an M-sequence. Each SSS sequence isgenerated by interleaving two SSC 1 sequence and SSC 2 sequence with alength of 31 in the frequency domain in an alternate fashion. At thistime, the SSC 1 sequence and the SSC 2 sequence are generated as adifferent cyclic shift value is applied to the M-sequence of length 31.At this time, the cyclic shift index is determined by a function ofphysical layer cell ID groups.

Carrier Aggregation

A communication environment considered in the embodiments of the presentinvention includes all multi-carrier environments. That is, amulti-carrier system or a carrier aggregation (CA) system used in thepresent invention refers to a system for aggregating and utilizing oneor more component carriers having a bandwidth smaller than a targetbandwidth, for wideband support.

In the present invention, multi-carrier refers to carrier aggregation.Carrier aggregation includes aggregation of contiguous carriers andaggregation of non-contiguous carriers. In addition, the number ofcomponent carriers aggregated in downlink and uplink may be differentlyset. The case where the number and/or bandwidth of downlink componentcarriers (DL CCs) and the number and bandwidth of uplink componentcarriers (UL CCs) are the same is referred to as symmetric aggregationand the case where the number and/or bandwidth of downlink componentcarriers (DL CCs) and the number and bandwidth of uplink componentcarriers (UL CCs) are different is asymmetric aggregation. Such carrieraggregation is used interchangeable with the terms “carrieraggregation”, “bandwidth aggregation” or “spectrum aggregation”.

Carrier aggregation configured by aggregating two or more CCs aims atsupport a bandwidth of up to 100 MHz in an LTE-A system. When one ormore carriers having a bandwidth smaller than a target bandwidth areaggregated, the bandwidth of the aggregated carriers may be restrictedto a bandwidth used in the existing system, for backward compatibilitywith the existing IMT system. For example, the existing 3GPP LTE systemmay support bandwidths of 1.4, 3, 5, 10, 15 and 20 MHz and anLTE_Advanced (LTE_A) system evolved from the LTE system may support abandwidth greater than 20 MHz using only the bandwidths supported by theLTE system. Alternatively, the carrier aggregation system used in thepresent invention may define a new bandwidth so as to support CA,regardless of the bandwidths used in the existing system.

The above-described carrier aggregation environment may be called amultiple-cell environment. The cell is defined as a combination ofdownlink resources (DL CCs) and uplink resources (UL CCs), and theuplink resources are not mandatory. Accordingly, the cell may becomposed of downlink resources alone or both downlink resources anduplink resources. If a specific UE has one configured serving cell, theUE may have one DL CC and one UL CC. If a specific UE has two or moreconfigured serving cells, the UE may have DL CCs corresponding in numberto the number of cells and the number of UL CCs may be equal to or lessthan the number of DL CCs, and vice versa. If a specific UE has aplurality of configured service cells, a carrier aggregation environmentin which the number of DL CCs is greater than the number of UL CCs mayalso be supported. That is, carrier aggregation may be regarded asaggregation of two or more cells having different carrier frequencies(center frequencies of a cell). If carrier aggregation is supported,linkage between a carrier frequency (or a DL CC) of downlink resourcesand a carrier frequency (or a UL CC) of uplink resources may beindicated by system information. The DL CC and the UL CC may be referredto as DL cell and UL cell, respectively. The cell described hereinshould be distinguished from a “cell” as a general region covered by aBS.

A cell used in the LTE-A system includes a primary cell (PCell) and asecondary cell (SCell). The PCell and the SCell may be used as servicecells. In case of a UE which is in an RRC_connected state but does notset carrier aggregation or supports carrier aggregation, only oneserving cell composed of a PCell exists. In contrast, in case of a UEwhich is in an RRC_CONNECTED state and sets carrier aggregation, one ormore serving cells exist. The serving cell includes a PCell and one ormore SCell.

A serving cell (PCell and SCell) may be set through an RRC parameter.PhyCellId is a physical layer identifier of a cell and has an integervalue from 0 to 503. SCellIndex is a short identifier used to identifyan SCell and has an integer value from 1 to 7. A value of 0 is appliedto the PCell and SCellIndex is previously given to be applied to theScell. That is, a cell having a smallest cell ID (or a cell index) inServCellIndex becomes the PCell.

The PCell refers to a cell operating on a primary frequency (e.g., aprimary CC (PCC)). The PCell is used to perform an initial connectionestablishment process or a connection re-establishment process at a UE.The PCell may indicate a cell indicated in a handover process. The PCellrefers to a cell for performing control-associated communication amongserving cells set in a carrier aggregation environment. That is, a UEmay receive a PUCCH allocated by a PCell to which the UE belongs andperform transmission and use only the PCell to acquire systeminformation and change a monitoring procedure. In evolved universalterrestrial radio access (E-UTRAN), a UE supporting a carrieraggregation environment may change only the PCell for a handoverprocedure using an RRCConnectionReconfiguration message of a higherlayer including mobilityControlInfo.

The SCell refers to a cell operating on a secondary frequency (e.g., asecondary CC (SCC)). Only one PCell may be allocated to a specific UEand one or more SCells may be allocated to the specific UE. The SCellmay be configured after radio resource control (RRC) connectionestablishment and may be used to provide additional radio resources. APUCCH is not present in cells except for the PCell among serving cellsset in a carrier aggregation environment, that is, the SCells. E-UTRANmay provide all system information associated with the operation of anassociated cell in an RRC_CONNECTED state via a dedicated signal whenSCells are added to a UE supporting a carrier aggregation environment.Change of system information may be controlled by release and additionof the SCell. At this time, an RRCConnectionReconfiguration message of ahigher layer may be used. The E-UTRAN may transmit a dedicated signalhaving a different parameter to each UE, rather than broadcasting asignal in the associated SCell.

After an initial security activation process begins, an E-UTRAN mayconfigure a network by adding one or more SCells to a PCell initiallyconfigured in a connection establishment process. In a carrieraggregation environment, the PCell and the SCell may operate asrespective CCs. In the following embodiments, a primary CC (PCC) may beused as the same meaning as the PCell and a secondary CC (SCC) may beused as the meaning as the SCell.

FIG. 8 represents an example of component carrier and carrieraggregation in the wireless communication system to which the presentinvention can be applied.

FIG. 8 (a) represents a single carrier structure that is used in a LTEsystem. There are DL CC and UL CC in component carrier. One componentcarrier may have 20 MHz frequency range.

FIG. 8 (b) represents a carrier aggregation structure that is used in aLTE-A system. FIG. 8 (b) represents a case that three component carriershaving 20 MHz frequency are aggregated. There are three DL CCs and ULCCs respectively, but the number of DL CCs and UL CCs are not limitedthereto. In case of the carrier aggregation, the UE enables to monitorthree CCs at the same time, to receive the DL signal/data, and totransmit the UL signal/data.

If, N DL CCs are managed in a specific cell, the network may allocate M(M≤N) DL CCs. In this case, the UE may monitor the limited M DL CCs onlyand receive the DL signal. Also, the network may give a priority to L DLCCs and have the prioritized DL CCs allocated to the UE, in this case,the UE should monitor the DL CCs without fail. This way may be appliedfor the UL transmission.

The linkage between the DL resource carrier frequency (or DL CC) and theUL resource carrier frequency (or UL CC) may be instructed by a higherlayer message like RRC message or system information. For example, thecombination of DL resource and UL resource may be configured by thelinkage that is defined by system information block type 2 (SIB2).Particularly, the linkage may signify the mapping relationship betweenthe DL CC through which the PDCCH carrying a UL grant is transmitted andthe UL CC that uses the UL grant, or signify the mapping relationshipbetween the DL CC (or UL CC) through which the data for HARQ istransmitted and the UL CC (or DL CC) through which the HARQ ACK/NACKsignal is transmitted.

FIG. 9 illustrates one example of carrier aggregation in a wirelesscommunication system to which the present invention can be applied.

FIG. 9(a) illustrates aggregation of contiguous carriers (namely, F1,F2, and F3), and FIG. 9(b) illustrates aggregation of non-contiguouscarriers (namely, F1, F2, and F3).

With reference to FIG. 9, there is no need for component carriers set upfor carrier aggregation to be contiguous to each other in the frequencydomain. Therefore, a network operator can provide a high data rateservice requiring a broad band by using fragmented spectrum withoutrelying on homogeneous broadband spectrum allocation.

Also, carrier aggregation can be classified into intra-band aggregationwithin the same band and inter-band aggregation and should be understoodto refer to both of the two cases.

Aggregation of contiguous carriers can correspond to intra-bandaggregation within the same band. On the other hand, aggregation ofnon-contiguous carriers can correspond not only to intra-bandaggregation within the same band but also to inter-band aggregation.

For each cell involved in carrier aggregation, capability is defined ina cell-specific manner, which indicates the number of carriers availablefor the cell. How to use the available carriers can be determined in aUE-specific manner. In other words, incase three carriers, F1, F2, F4,are available in a cell, a specific UE may use F1 and F2 through carrieraggregation, while another UE may use F2 and F4 through carrieraggregation.

Cross Carrier Scheduling

In case a particular UE aggregates one or more carriers, a PCell and anSCell are defined for the UE to operate the corresponding carriers. Inother words, carriers assigned to a PCell should always operate in anactivated state, while those carriers assigned to an SCell may beactivated or deactivated depending on the needs. At this time, withrespect to a UE for which more than one SCell is activated, schedulingof data transmitted to the SCell can be carried out as follows.

In a carrier aggregation system, in view of scheduling of carriers (orcarrier waves) or serving cells, two methods can be employed: aself-scheduling method and a cross carrier scheduling method. Crosscarrier scheduling can be called cross component carrier scheduling orcross cell scheduling.

In cross carrier scheduling, a PDCCH (DL Grant) and a PDSCH aretransmitted to different DL CCs, or a PUSCH transmitted according to thePDCCH (UL Grant) transmitted from the DL CC is transmitted to adifferent UL CC rather than the UL CC linked to the DL CC which hasreceived the UL grant.

Cross carrier scheduling can be activated or deactivated in aUE-specific manner and can be notified to each UE in a semi-staticmanner through upper layer signaling (for example, RRC signaling).

In case cross carrier scheduling is activated, a carrier indicator field(CIF) is needed, which informs a PDCCH about through which DL/UL cc thePDSCH/PUSCH indicated by the corresponding PDCCH is transmitted. Forexample, a PDCCH can allocate PDSCH resources or PUSCH resources to oneof a plurality of component carriers by using the CIF. In other words,the CIF is set when a PDCCH on a DL CC allocates PDSCH or PUSCHresources to one of multi-aggregated UL/UL CCs. In this case, the DCIformat of the LTE-A release-8 can be extended according to the CIF. Thenthe CIF can be fixed as a 3 bit field, or location of the CIF can befixed independently of the size of the DCI format. Also, the PDCCHstructure of the LTE-A release-8 (resource mapping based on the samecoding and the same CCE) may be re-used.

When cross carrier scheduling is activated, it is necessary for a UE tomonitor a PDCCH with respect to a plurality of DCIs in the controlregion of a monitoring CC according to a transmission mode and/orbandwidth for each CC. Therefore, along with PDCCH monitoring, it isnecessary to construct a search space to support the PDCCH monitoring.

In a carrier aggregation system, a UE DL CC set represents a set of DLCCs scheduled so that a UE can receive a PDSCH, while a UE UL CC setrepresents a set of UL CCs scheduled so that a UE can transmit a PUSCH.Also, a PDCCH monitoring set represents a set consisting of at least oneDL CC which carries out PDCCH monitoring. A PDCCH monitoring set may bethe same as the UE DL CC set or a subset of the UE DL CC set. The PDCCHmonitoring set can include at least one of the DL CCs of the UE DL CCset. Or the PDCCH monitoring set can be defined independently of the UEDL CC set. A DL CC included in the PDCCH monitoring set can beconfigured so that self-scheduling with respect to a UL CC linked to theDL CC is always possible. The UE DL CC set, UE UL CC set, and PDCCHmonitoring set can be configured in a UE-specific, in a UEgroup-specific, or in a cell-specific manner.

In case cross carrier scheduling is deactivated, it indicates that aPDCCH monitoring set is always the same as a UE DL CC set; in this case,an indication such as separate signaling with respect to the PDCCHmonitoring set is not needed. However, in case cross carrier schedulingis activated, it is preferable that the PDCCH monitoring set is definedwithin the UE DL CC set. In other words, to schedule a PDSCH or a PUSCHwith respect to a UE, an eNB transmits a PDCCH only through the PDCCHmonitoring set.

FIG. 10 illustrates one example of a subframe structure according tocross carrier scheduling in a wireless communication system to which thepresent invention can be applied.

Referring to FIG. 10, three DL CCs are concatenated in a DL subframeintended for a LTE-A UE, and the DL CC ‘A’ is configured to be a PDCCHmonitoring DL CC. In case CIF is not used, each DL CC can transmit aPDCCH which schedules its own PDSCH without employing a CIF. On theother hand, in case the CIF is used through upper layer signaling, onlythe DL CC ‘A’ can transmit a PDCCH which schedules its own PDSCH or aPDSCH of another CC by using the CIF. At this time, the DL CC ‘B’ and‘C’ not configured as a PDCCH monitoring DL CC do not transmit a PDCCH.

2-Level Radio Frame Structure and a Method for Transmitting andReceiving Data

The LTE/LTE-A system has a frame structure having a 1 ms transmissiontime interval (TTI), and in most cases, requested delay time of data forvideo applications is approximately 10 ms.

However, future 5G technology is facing demand for a much lower delay indata transmission due to advent of new applications such as real-timecontrol and tactile Internet, and it is expected that the required delaytime for data transmission in the 5G technology will be reduced down toabout 1 ms.

However, the existing frame structure having a 1 ms TTI has an inherentproblem that the requirement of 1 ms delay for data transmission cannotbe met.

FIG. 11 illustrates a delay in wireless transmission and reception inthe 3GPP LTE/LTE-A system to which the present invention can be applied.

FIG. 11 illustrates a reference delay in wireless transmission andreception in view of downlink transmission and reception of the 3GPP LTEsystem having a 1 ms subframe.

Referring to FIG. 11, a propagation delay (PD) is generated between thetime the eNB starts transmission of a downlink subframe and the time theUE starts receiving the downlink subframe. And a buffering delay isoccurred as the UE buffers the downlink subframe before decoding thedownlink subframe. A delay due to a propagation delay with respect todownlink subframe transmission and buffering in the UE amounts to atotal of approximately 0.5 ms. And the UE decodes a PDCCH in a downlinksubframe and decodes a PDSCH based on the PDCCH decoding information. Aprocessing delay due to the PDCCH decoding (approximately 0.5 ms) andthe PDSCH delay (less than approximately 2 ms) measures to be less thanapproximately 2.5 ms.

In this manner, the one-way over-to-air (OTA) latency from the eNB tothe UE becomes less than approximately 3 ms.

And a delay for ACK/NACK (A/N) preparation in the UE (for example,ACK/NACK encoding) and a propagation delay generated at the time oftransmitting the A/N require less than a total of approximately 1 ms.

In this way, for one way data transmission, approximately 4 ms isnormally required for a total roundtrip OTA latency from atransmitter-side (for example, the eNB) until the ACK/NACK is receivedat a receiver-side (for example, the UE).

The 5G wireless communication system aims to provide a data delayreduced by about ten times of the existing wireless communicationsystem. To achieve the goal, the 5G system is expected to adopt a newframe structure having a shorter TTI (for example, 0.2 ms).

It is also anticipated that the 5G system is confronted with not onlythe low latency but also applications demanding various requirementssuch as high capacity, low energy consumption, low cost, and high userdata rate. Thus, the 5G system is expected to evolve to such a systemdifferent from the existing ones to support various kinds ofapplications ranging from the ones demanding ultra-low latency to thosedemanding a high data rate.

Therefore, to minimize a data reception delay in a UE, a new framestructure different from those of the existing wireless communicationsystems needs to be defined, and an effect on legacy UEs due to the newframe has to be minimized.

The present invention, to provide a user with various services demandingdisparate requirements, proposes a system which provides more than oneframe structure to a particular UE.

In other words, by defining a frame structure for each sub-band (or asub-band group or a band/carrier), the present invention defines morethan one service-specific sub-band (or sub-band group or aband/carrier). For example, the present invention supports so that theexisting 1 ms TTI frame structure for ordinary data transmission and ashort TTI frame structure for data transmission demanding low latencycan be employed for a particular UE.

In what follows, a short TTI can be understood to have the same meaningas a short TTI subframe (or a short subframe). That is to say, in caseboth of the control region and the data region are defined in one shortsubframe, the short TTI has the size covering both of the control andthe data region, while, in case only the data region is defined withinthe short subframe, the short TTI has the size covering only the dataregion.

In what follows, for the convenience of descriptions, a radio framestructure employing normal CP of the FDD type according to an embodimentof the present invention will be described. It should be noted, however,that the present invention is not limited to the aforementionedembodiment but can be applied in the same way for a radio framestructure of the TDD type or a radio frame structure employing extendedCP.

Subband-Wise 2-Level Frame Structure

The 3GPP LTE/LTE-A system defines a sub-band as a set of resource blocks(RBs). N_(RB) ^(sb) represents the size of each sub-band and denotes thenumber of RBs. N_(RB) ^(sb) can be calculated by the Equation 1 below.

$\begin{matrix}{N_{RB}^{sb} = \left\{ \begin{matrix}N_{RB}^{UL} & {N_{sb} = 1} \\\left\lfloor {\left( {N_{RB}^{UL} - N_{RB}^{HO} - {N_{RB}^{HO}{mod}\; 2}} \right)/N_{sb}} \right\rfloor & {N_{sb} > 1}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In Equation 1, N_(sb) the number of sub-bands and is determined by anupper layer.

N_(RB) ^(UL) represents uplink bandwidth configuration and is expressedby resource block size (namely, the number of sub-carriers per resourceblock, N_(sc) ^(RB)).

N_(RB) ^(HO) represents an offset used for frequency hopping(‘pusch-HoppingOffset’) and is expressed by the number of resourceblocks. N_(RB) ^(HO) and hopping-mode related parameters (namely,inter-subframe or intra and inter-subframe) are determined by an upperlayer.

The equation above illustrates an example where a sub-band is calculatedbased on uplink bandwidth, but the present invention can define morethan one sub-band similarly for downlink bandwidth and/or uplinkbandwidth.

As described above, one downlink and/or uplink band (namely, a carrieror a cell) can be divided into a plurality of sub-bands. In whatfollows, described will be a method for composing more than one sub-band(or sub-band group or band/carrier) within one downlink and/or uplinkband by employing a short TTI frame structure.

In what follows, for the sake of convenience, the method will bedescribed with respect to a downlink band (namely, a carrier or a cell).

FIG. 12 illustrates a radio frame structure according to one embodimentof the present invention.

With reference to FIG. 12, a conventional PDCCH 1201 is allocated to amaximum of four symbols for each legacy subframe. In other words, theconventional PDCCH can be transmitted across the whole band through amaximum of #0 to #3 symbol of each subframe. FIG. 12 assumes that thePDCCH 1201 is allocated across the whole band through #0 and #1 symbolof each subframe.

In what follows, for the convenience of descriptions, it is assumed thatthe PDCCH 1201 is mapped to the first two symbols for each subframe.

In the frequency region except for a sub-band dedicated to low latency,a PDSCH 1202 for normal data transmission is allocated to the remainingsymbols to which the PDCCH 1201 is not mapped. FIG. 12 illustrate a casewhere a PDSCH 1202 is allocated across the whole frequency region exceptfor a sub-band dedicated to low latency through #3 to #13 symbol.

And for an arbitrary band, more than one sub-band (or a sub-band group)for low latency can employ a short TTI frame structure.

In other words, in the case of a legacy subframe, more than one sub-band(or sub-band group) subdivides the symbols excluding those symbols towhich the PDCCH 1201 has been mapped (namely, the whole symbols of thelegacy subframe except for the symbol to which the PDCCH has beenallocated) into n symbols of which the size corresponds to the size of ashort TTI (for example, 2 to 4 symbol) and is composed of short TTIsubframes sPDSCHs 1203. In this case, since only the sPDSCH is allocatedto the short TTI subframe, the short TTI subframe and the sPDSCH can beinterpreted in the same manner.

As shown in FIG. 12, in case the PDCCH 1201 is allocated to theaforementioned two symbols of a legacy subframe, four (=12/3) shortsubframes (sPDSCH) 1203 can be composed.

In this way, in case a subframe employs a short TTI frame structure, ashort resource block (RB) can also be newly defined as a resourceallocation unit for a low latency UE. For example, a short RB can bedefined such that it consists of 12 sub-carriers in the frequency domainin the same way as in the prior art but consists of n symbols (namely,symbols having a short TTI size) in the time domain. Also, the short RBmay consist of x sub-carriers (x<12), of which the total number issmaller than in the prior art, even in the frequency domain.

In the same manner as in the legacy PDSCH, mapping of data onto resourceelements in the sPDSCH region can be first carried out in the increasingorder of frequency index and then in the increasing order of symbolindex.

The symbols (or the number of symbols) to which the PDCCH 1201 andsPDSCH 1203 are mapped; and the number of short TTIs employed within onelegacy subframe described earlier are only an example and the presentinvention is not limited to the example above.

In case a subframe comprises a short TTI frame structure as illustratedin FIG. 12, control information with respect to downlink datatransmitted from a short TTI subframe (sPDSCH) 1203 (for example,frequency/time resource allocation information of the sPDSCH withrespect to the downlink data, modulation and coding scheme (MCS), newdata indicator (NDI), redundancy version (RV), transmit power control(TPC) command, and so on) can be transmitted through the PDCCH 1201.

As described above, in case the eNB transmits control information aboutdownlink data transmitted from the sPDSCH 1203 through the PDCCH 1201,the PDCCH 1201 transmits control information related to the sPDSCH 1203as well as the PDSCH 1202. Therefore, the UE needs to know from which ofthe PDSCH 1202 and the short TTI subframe (sPDSCH) 1203 downlink dataare transmitted to the UE.

To this end, the downlink control information (DCI) format related todownlink data transmission can include a ‘sPDSCH indicator field’.

Also, the DCI format related to downlink data transmission can include a‘TTI number field’ indicating which short TTI subframe 1203 among 12/nshort TTI subframes 1203 is scheduled to receive data.

In the following, detailed information about each field according to thepresent invention is provided.

1) sPDSCH Indicator (1 Bit)

According to the present invention, the DCI related to downlink datatransmitted from an sPDSCH includes an sPDSCH indicator (for example, 1bit).

Also, the sPDSCH indicator (for example, 1 bit) may be added to the DCIformat (in other words, the DCI format 1, 1A, 1B, 1C) for scheduling ofdownlink data of the existing PDCCH.

In case a particular band employs a sub-band specific 2-level framestructure according to the present invention, the UE has to performdecoding of data by using a short subframe structure in order tosuccessfully receive data transmitted through an sPDSCH having a shortTTI.

Therefore, for successful data reception, at the time of receiving aPDCCH, the UE needs to know whether the corresponding data are comingthrough the existing PDSCH or through an sPDSCH. To distinguish the twocases from each other, the DCI format transmitted through the PDCCH caninclude an one-bit sPDSCH indicator.

2) TTI Number (m Bit, for Example, 2 Bit in the Case of a 3 Symbol TTI)

In case one TTI comprises n symbols (for example, 3), the UE should beable to identify the region (namely, TTI/sPDSCH) to which its owndownlink data are transmitted.

To this end, the DCI related to downlink data transmitted from an sPDSCHcan include a TTI number field indicating to which TTI among 12/n (forexample, 4) short TTIs the downlink data are transmitted.

Also, the DCI format for scheduling of downlink data of the existingPDCCH (namely, DCI format 1, 1A, 1B, 1C) may include the TTI numberfield.

The TTI number field can has a length of m bits (for example, in case nis 3, the length of the TTI number field meant for distinguishing 4 TTIsfrom each other is 2 bits). In this case, the TTI number field can beused to notify of the number of an sPDSCH within 1 ms (namely, legacysubframes).

In case a TTI comprises 3 symbols, 4 sPDSCHs can exist within 1 ms, andto identify the individual sPDSCHs, a TTI number field having a lengthof 2 bits can be included in a downlink grant (DL grant). In otherwords, the values of the corresponding fields are 0b00 for the 0-thsPDSCH, 0b01 for the 1st sPDSCH, 0b10 for the 2nd sPDSCH, and 0b11 forthe 3rd sPDSCH.

Meanwhile, the sPDSCH indicator information and the TTI numberinformation described above may be combined into one field.

In other words, the sPDSCH indicator and the TTI number field defined inthe form of a bitmap may transmit all of the sPDSCH indicatorinformation and the TTI number information.

For example, in case a TTI comprises 3 symbols, 4 sPDSCHs can existwithin 1 ms, and to identify the individual sPDSCHs, an sPDSCH indicatorhaving a length of 4 bits and a TTI number field can be incorporatedinto a DL grant of the existing PDCCH. In other words, values of thecorresponding fields are 1000 for the 0-th sPDSCH, 0100 for the 1stsPDSCH, 0010 for the 2nd sPDSCH, and 0001 for the 3rd sPDSCH. If thesPDSCH indicator and the TTI number field are set up by one of thevalues, the UE can know that downlink data are transmitted to thesPDSCH.

On the other hand, if the sPDSCH indicator and the TTI number field areall set to ‘0000’, it indicates that downlink data for the correspondingUE are not transmitted through the sPDSCH but transmitted through thePDSCH.

FIG. 13 illustrates a radio frame structure according to one embodimentof the present invention.

With reference to FIG. 13, the PDCCH 1301 is allocated to the maximum of4 aforementioned symbols for each subframe. In other words, the PDCCHcan be transmitted across the whole band through a maximum of #0 to #3symbols. FIG. 13 assumes the case where the PDCCH 1301 is composedacross the whole band through #0 and #1 symbol of each subframe.

In what follows, for the convenience of descriptions, it is assumed thatthe PDCCH 1301 is mapped to the first two symbols for each subframe.

In the frequency region except for a sub-band dedicated to low latency,a PDSCH 1302 for normal data transmission can be allocated to theremaining symbols to which the PDCCH 1301 is not mapped. FIG. 13illustrate a case where a PDSCH 1302 is allocated across the wholefrequency region except for a sub-band dedicated to low latency through#3 to #13 symbol.

And more than one sub-band (or a sub-band group) for low latency canemploy a short TTI frame structure. More specifically, in the case of alegacy subframe, more than one sub-band (or sub-band group) subdividesthe symbols excluding those symbols to which the PDCCH 1301 has beenmapped (namely, the whole symbols of the legacy subframe except for thesymbol to which the PDCCH 1301 has been allocated) into n symbols ofwhich the size corresponds to the size of a short TTI (for example, 2 to4 symbol) and is composed of short TTI subframes (sPDSCHs 1303 andsPDSCHs 1304).

In other words, for each short TTI subframe, the sPDCCH 1303 isallocated to preceding, predetermined symbols (for example, one or twosymbols), and the sPDCCH 1304 is allocated to the remaining symbols.

Although the number of symbols for the sPDCCH 1303 is not limited, it ispreferred that the sPDCCH 1303 is composed of one symbol in case a shortTTI subframe consists of three symbols.

As shown in FIG. 13, in case the PDCCH 1301 is allocated to theaforementioned two symbols of a legacy subframe, each subframe cancomprise four (=12/3) short TTI subframes (sPDCCH 1301 and sPDCCH 1304).And for each short TTI subframe, the sPDCCH 1303 is allocated to thepreceding one symbol, and the sPDSCH 1304 can be allocated to theremaining two symbols.

As described above, in case a subframe employs a short TTI framestructure, a short resource block (RB) can also be newly defined as aresource allocation unit for a low latency UE. For example, a short RBcan be defined such that it consists of 12 sub-carriers in the frequencydomain in the same way as in the prior art but consists of n symbols(namely, symbols having a short TTI size) in the time domain. Also, theshort RB may consist of x sub-carriers (x<12), of which the total numberis smaller than in the prior art, even in the frequency domain.

Also, in the same manner as in the legacy PDSCH, mapping of data ontoresource elements in the sPDCCH region can be first carried out in theincreasing order of frequency index and then in the increasing order ofsymbol index.

The symbols (or the number of symbols) to which the PDCCH 1301, sPDSCH1303, and sPDSCH 1304 are mapped; and the number of short TTIs employedwithin one legacy subframe described earlier are only an example and thepresent invention is not limited to the example above.

As shown in FIG. 13, in case a subframe comprises short TTI subframes(sPDCCHs 1303 and sPDSCHs 1304), downlink control information (forexample, frequency/time resource allocation information with respect tothe downlink data of the sPDSCH; MCS, NDI, RV, TPC command; and so on)with respect to the downlink data transmitted from the sPDSCH 1304 canbe transmitted through the sPDCCH 1303 newly defined within the shortTTI subframe. In other words, the sPDCCH 1303 is transmitted across thewhole band from a predetermined symbol within the short TTI subframe.

The PDCCH 1301 is transmitted to a set of one or more contiguous controlchannel elements (CCEs). A CCE consists of 9 resource element groups(REGs), and an REG consists of 4 resource elements.

However, in case the sPDCCH 1303 is configured as shown in FIG. 13, theformat of the sPDCCH 1303 can be the same as that of the PDCCH 1301,which can be defined by a different format. For example, in the sPDCCH1303 format, one CCE can consist of x (x<9) REGs, and the REG mapped tothe sPDCCH 1303 region can consist of y (y<4) REs.

Meanwhile, the present invention can employ not only the new radio framestructure for low latency as described in FIGS. 12 and 13 but also aframe structure developed for a purpose different from the 1 ms subframestructure defined in the existing LTE/LTE-A standard (for example,transmission of data generated by an application requiring low latency)in the same band. In what follows, the frame structure according to thepresent invention will be described with reference to accompanyingdrawings.

FIG. 14 illustrates a radio frame structure according to one embodimentof the present invention.

With reference to FIG. 14, physical channels of the same frame structurecan be designed for each sub-band (or sub-band group). In this case, aPDCCH can be formed for each sub-band (or sub-band group).

In other words, a PDCCH allocated to the corresponding sub-band (orsub-band group) transmits control information (for example,frequency/time resource allocation information with respect to thedownlink data of the PDSCH or the sPDSCH; MCS, NDI, RV, TPC command; andso on) for a PDSCH allocated to the corresponding sub-band.

This scheme can be used for such a case where a radio frame structure ofthe existing LTE/LTE-A system is employed and data generated by aservice suitable for low latency are transmitted through a particularsub-band.

For example, the sub-band (or sub-band group) #1 can be configured totransmit normal data, while the sub-band (or sub-band group) #2 issupposed to transmit data generated by an application requiring lowlatency.

At this time, a control channel allocated to the control region of asub-band (or sub-band group) for transmitting data generated by anapplication requiring low latency may be called an sPDCCH, while a datachannel allocated to the data region may be called a PDSCH.

As shown in FIG. 14, in case data intended for disparate purposes aretransmitted for each sub-band (or sub-band group), the format of thePDCCH (or sPDCCH) transmitted from a sub-band (or sub-band group) totransmit data generated by an application requiring low latency can bethe same as the format of the existing PDCCH, but may be defined by theone different from that of the existing PDCCH. For example, one CCE inthe PDCCH (or sPDCCH) transmitted from a sub-band (or sub-band group)for transmitting data generated by an application requiring low latencycan be composed of x REGs (x<9), or an REG can be composed of y REs(y<4).

Carrier/Cell-Wise 2-Level Frame Structure

The present invention can also be applied as a carrier/cell-wise 2-levelframe structure rather than the subband-wise 2-level frame structure.

In other words, a carrier compliant with the existing LTE/LTE-A framestructure and a carrier compliant with a new radio frame structure canbe allocated to a particular user.

This scheme can be used as another method for carrier aggregation. Inthis case, it is preferred that a PCell (Primary Cell) is configured tobe compliant with the existing LTE/LTE-A frame structure, but a cellcompliant with a new frame structure may be configured to operate as aPCell depending on a UE's characteristics. For a UE requiring lowlatency, by forcing two cells having frame structures different fromeach other to be aggregated, both of the two cells may be configured tooperate as a PCell.

FIG. 15 illustrates a radio frame structure according to one embodimentof the present invention.

With reference to FIG. 15, the band (or carrier/cell) 1 is set upaccording to a legacy frame structure, the band (or carrier/cell) 2 isset up according to the short TTI frame structure of the presentinvention, and the band 1 and 2 are carrier-aggregated with respect to aparticular UE.

In the band 1, a PDCCH 1501 and a PDSCH 1502 can be mapped according tothe definition of the existing LTE/LTE-A system. In other words, thePDCCH 1501 is allocated to the maximum of 4 first symbols for eachlegacy subframe. In other words, the PDCCH can be transmitted across thewhole band 1 through a maximum of #0 to #3 symbols. FIG. 15 assumes thecase where the PDCCH 1501 is allocated across the whole band through #0and #1 symbol of each subframe.

For those symbols to which the PDCCH 1501 is not mapped, the PDSCH 1502used for transmission of normal data can be allocated across the wholeband 1. FIG. 15 illustrates the case where the PDSCH 1502 is allocatedacross the whole band 1 for #3 to #13 symbols.

The band 2 having a short TTI frame structure can comprise more than onespecial symbol 1503 within one legacy subframe (namely, 1 ms) and morethan one sPDSCH 1504 having the size of n symbols.

FIG. 15 illustrates the case where a special symbol 1503 having the sizeof one symbol and two sPDSCHs 1504 having the size of 3 symbols (n=3)are mapped; and a special symbol 1503 having the size of one symbol andtwo sPDSCHs 1504 having the size of 3 symbols (n=3) are subsequentlymapped within one legacy frame.

In other words, a short TTI radio frame structure comprises four shortTTIs within one legacy subframe (namely, 1 ms), and one short TTI has alength of 3 symbols (namely, the length of an sPDSCH), which is about alength of 0.2 ms.

At this time, the special symbol 1503 can be composed of the symbolscorresponding to the remainder (2=14% 3) of dividing the total number ofsymbols by the size of a short TTI (3 symbol in the case of FIG. 15). Atthis time, the order by which the special symbol 1503 and the sPDSCH1504 are mapped onto the time axis can differ from the order asillustrated in FIG. 15.

As shown in FIG. 15, in case a band is composed of the special symbol1503 and the sPDSCH 1504 only, the control information (for example,frequency/time resource allocation information with respect to thedownlink data of the PDSCH or the sPDSCH; MCS, NDI, RV, TPC command; andso on) with respect to the downlink data transmitted from the sPDSCH1504 can be transmitted through the PDCCH 1501 of another band (band 1in the case of FIG. 15). In other words, cross carrier scheduling can beapplied.

As described above, in case the eNB transmits control information withrespect to the downlink data transmitted from the sPDSCH 1504 throughthe PDCCH 1501, the PDCCH 1501 transmits control information related tothe sPDSCH 1504 of the band 2 as well as the PDSCH 1502 of the band 1.Therefore, the UE needs to know from which of the PDSCH 1502 and thesPDSCH 1504 the downlink data are transmitted to the UE.

To this end, as described in the example of FIG. 12, the downlinkcontrol information (DCI) format related to downlink data transmissioncan include a ‘sPDSCH indicator field’ and/or a ‘TTI number field’indicating which sPDSCH 1504 among 12/n sPDSCHs 1504 is scheduled toreceive data.

Also, the sPDSCH indicator information and the TTI number informationmay be combined into one field. In other words, the sPDSCH indicator andthe TTI number field defined in the form of a bitmap may transmit all ofthe sPDSCH indicator information and the TTI number information.

As described above, in case a subframe employs a short TTI framestructure, a short resource block (RB) can also be newly defined as aresource allocation unit for a low latency UE. For example, a short RBcan be defined such that it consists of 12 sub-carriers in the frequencydomain in the same way as in the prior art but consists of n symbols(namely, symbols having a short TTI size) in the time domain. Also, theshort RB may consist of x sub-carriers (x<12), of which the total numberis smaller than in the prior art, even in the frequency domain.

Also, in the same manner as in the legacy PDSCH, mapping of data ontoresource elements in the sPDSCH region can be first carried out in theincreasing order of frequency index and then in the increasing order ofsymbol index.

Meanwhile, in the case of FIG. 15, the sPDCCH 1503 which transmitscontrol information related to the sPDSCH 1504 may be allocated insteadof the special symbol 1503.

In this case, in the band 2 which employs a short TTI frame structure,the sPDCCH 1503 can be mapped to more than one sPDSCH 1504. As shown inFIG. 15, the first and the second sPDSCH 1504 from the left of a legacysubframe are mapped to the first sPDCCH 1503, while the third and thefourth sPDSCH 1504 are mapped to the second sPDCCH 1503.

Therefore, control information (for example, frequency/time resourceallocation information with respect to the downlink data of the sPDSCH;MCS, NDI, RV, TPC command; and so on) with respect to the downlink datatransmitted from the sPDSCH 1504 can be transmitted through the sPDSCH1504 and the sPDCCH being mapped 1503 (namely, the sPDCCH transmittedmost recently before the corresponding sPDSCH). At this time, the sPDCCH1503 is transmitted across the whole band.

Although the format of the sPDCCH 1503 employing a short TTI framestructure can be the same as that of the existing PDCCH 1501, it can bedefined by a different format. For example, in the sPDCCH 1503 format,one CCE can consist of x (x<9) REGs, and the REG mapped to the sPDCCH1503 region can consist of y (y<4) REs.

As described above, the band 1 employing a legacy frame structure or theband 2 employing a short TTI frame structure can operate as a PCell.Also, for a UE requiring low latency, by forcing the band 1 employing alegacy frame structure and the band 2 employing a short TTI framestructure to be aggregated, both of the band 1 and band 2 may beconfigured to operate as a PCell.

The symbols (or the number of symbols) to which the PDCCH 1501, sPDCCH1503, and sPDSCH 1504 are mapped; and the number of short TTIs employedwithin one legacy subframe described earlier are only an example and thepresent invention is not limited to the example above.

However, it is preferred that the size n of a short TTI (the number ofsymbols) should be smaller than 7 to design a frame structure providinglow latency.

FIG. 16 illustrates a radio frame structure according to one embodimentof the present invention.

With reference to FIG. 16, the band (or carrier/cell) 1 is set upaccording to a legacy frame structure, the band (or carrier/cell) 2 isset up according to the short TTI frame structure of the presentinvention, and the band 1 and 2 are carrier-aggregated with respect to aparticular UE.

Since the band 1 is the same as the example of FIG. 15, descriptionsrelated thereto will be omitted.

The band 2 having a short TTI frame structure can comprise more than onespecial symbol 1603 and more than one sPDSCH 1604 having the size of nsymbols within one legacy subframe (namely, 1 ms).

FIG. 16 illustrates the case where a special symbol 1603 having the sizeof two symbols and four contiguous sPDSCHs 1504 each of which having thesize of 3 symbols (n=3) are mapped within one legacy frame of the band2.

In other words, a short TTI radio frame structure comprises four shortTTIs within one legacy subframe (namely, 1 ms), and one short TTI has alength of 3 symbols (namely, the length of an sPDSCH), which is about alength of 0.2 ms.

At this time, the special symbol 1603 can be composed of the symbolscorresponding to the remainder (2 14% 3) of dividing the total number ofsymbols by the size of a short TTI (3 symbol in the case of FIG. 16). Atthis time, the order by which the special symbol 1603 and the sPDSCH1604 are mapped onto the time axis can differ from the order asillustrated in FIG. 16.

As shown in FIG. 16, in case a band is composed of the special symbol1603 and the sPDSCH 1604 only, the control information (for example,frequency/time resource allocation information with respect to thedownlink data of the sPDSCH; MCS, NDI, RV, TPC command; and so on) withrespect to the downlink data transmitted from the sPDSCH 1604 can betransmitted through the PDCCH 1501 of another band (band 1 in the caseof FIG. 16). In other words, cross carrier scheduling can be applied.

As described above, in case the eNB transmits control information withrespect to the downlink data transmitted from the sPDSCH 1604 throughthe PDCCH 1601, the PDCCH 1601 transmits control information related tothe sPDSCH 1604 of the band 2 as well as the PDSCH 1602 of the band 1.Therefore, the UE needs to know from which of the PDSCH 1602 and thesPDSCH 1604 the downlink data are transmitted to the UE.

To this end, as described in the example of FIG. 12, the downlinkcontrol information (DCI) format related to downlink data transmissioncan include a ‘sPDSCH indicator field’ and/or a ‘TTI number field’indicating which sPDSCH 1604 among 12/n sPDSCHs 1604 is scheduled toreceive data.

Also, the sPDSCH indicator information and the TTI number informationmay be combined into one field. In other words, the sPDSCH indicator andthe TTI number field defined in the form of a bitmap may transmit all ofthe sPDSCH indicator information and the TTI number information.

As described above, in case a subframe employs a short TTI framestructure, a short resource block (RB) can also be newly defined as aresource allocation unit for a low latency UE. For example, a short RBcan be defined such that it consists of 12 sub-carriers in the frequencydomain in the same way as in the prior art but consists of n symbols(namely, symbols having a short TTI size) in the time domain. Also, theshort RB may consist of x sub-carriers (x<12), of which the total numberis smaller than in the prior art, even in the frequency domain.

Also, in the same manner as in the legacy PDSCH, mapping of data ontoresource elements in the sPDSCH region can be first carried out in theincreasing order of frequency index and then in the increasing order ofsymbol index.

Meanwhile, in the case of FIG. 16, the sPDCCH 1603 which transmitscontrol information related to the sPDSCH 1604 may be allocated insteadof the special symbol 1603.

In this case, in the band 2 which employs a short TTI frame structure,the sPDCCH 1603 can be mapped to more than one sPDSCH 1604. As shown inFIG. 16, the first and the second sPDSCH 1604 from the left of a legacysubframe are mapped to the first sPDCCH 1603, while the third and thefourth sPDSCH 1604 are mapped to the second sPDCCH 1603.

Therefore, control information (for example, frequency/time resourceallocation information with respect to the downlink data of the sPDSCH;MCS, NDI, RV, TPC command; and so on) with respect to the downlink datatransmitted from the sPDSCH 1604 can be transmitted through thecorresponding sPDSCH 1604 and the sPDCCH being mapped 1603 (namely, thesPDCCH transmitted most recently before the corresponding sPDSCH). Atthis time, the sPDCCH 1603 is transmitted across the whole band.

Although the format of the sPDCCH 1603 employing a short TTI framestructure can be the same as that of the existing PDCCH 1601, it can bedefined by a different format. For example, in the sPDCCH 1603 format,one CCE can consist of x (x<9) REGs, and the REG mapped to the sPDCCH1603 region can consist of y (y<4) REs.

As described above, the band 1 employing a legacy frame structure or theband 2 employing a short TTI frame structure can operate as a PCell.Also, for a UE requiring low latency, by forcing the band 1 employing alegacy frame structure and the band 2 employing a short TTI framestructure to be aggregated, both of the band 1 and band 2 may beconfigured to operate as a PCell.

The symbols (or the number of symbols) to which the sPDCCH 1603 andsPDSCH 1604 are mapped; and the number of short TTIs employed within onelegacy subframe described earlier are only an example and the presentinvention is not limited to the example above.

However, it is preferred that the size n of a short TTI (the number ofsymbols) should be smaller than 7 to design a frame structure providinglow latency.

FIG. 17 illustrates a radio frame structure according to one embodimentof the present invention.

With reference to FIG. 17, the band (or carrier/cell) 1 is set upaccording to a legacy frame structure, the band (or carrier/cell) 2 isset up according to the short TTI frame structure of the presentinvention, and the band 1 and 2 are carrier-aggregated with respect to aparticular UE.

Since the band 1 is the same as the example of FIG. 15, descriptionsrelated thereto will be omitted.

The band 2 having a short TTI frame structure can comprise sPDCCHs 1703and sPDSCHs 1704 in an alternate fashion within more than one legacysubframe (namely, 3 ms).

FIG. 17 illustrates the case where an sPDCCH 1703 having the size of onesymbol and an sPDSCH 1704 having the size of 2 symbols (n=2) are mappedin an alternate fashion within three legacy frames of the band 2.

In other words, a short TTI radio frame structure comprises 14 shortTTIs within three legacy subframes (namely, 3 ms), and one short TTI hasa length of 3 symbols (namely, the length of an sPDCCH and an sPDSCH),which is about a length of 0.2 ms.

As described above, in case a subframe employs a short TTI framestructure, a short resource block (RB) can also be newly defined as aresource allocation unit for a low latency UE. For example, a short RBcan be defined such that it consists of 12 sub-carriers in the frequencydomain in the same way as in the prior art but consists of n symbols(namely, symbols having a short TTI size) in the time domain. Also, theshort RB may consist of x sub-carriers (x<12), of which the total numberis smaller than in the prior art, even in the frequency domain.

Also, in the same manner as in the legacy PDSCH, mapping of data ontoresource elements in the sPDSCH region can be first carried out in theincreasing order of frequency index and then in the increasing order ofsymbol index.

The control information (for example, frequency/time resource allocationinformation with respect to the downlink data of the sPDSCH; MCS, NDI,RV, TPC command; and so on) with respect to the downlink datatransmitted from the sPDSCH 1704 can be transmitted through thecorresponding sPDSCH 1704 and the sPDCCH being mapped 1703 (namely, thesPDCCH transmitted most recently before the corresponding sPDSCH). Atthis time, the sPDCCH 1703 is transmitted across the whole band.

Although the format of the sPDCCH 1703 employing a short TTI framestructure can be the same as that of the existing PDCCH 1701, it can bedefied by a different format. For example, in the sPDCCH 1703 format,one CCE can consist of x (x<9) REGs, and the REG mapped to the sPDCCH1703 region can consist of y (y<4) REs.

As described above, the band 1 employing a legacy frame structure or theband 2 employing a short TTI frame structure can operate as a PCell.Also, for a UE requiring low latency, by forcing the band 1 employing alegacy frame structure and the band 2 employing a short TTI framestructure to be aggregated, both of the band 1 and band 2 may beconfigured to operate as a PCell.

The symbols (or the number of symbols) to which the sPDCCH 1703 andsPDSCH 1704 are mapped; and the number of short TTIs employed within onelegacy subframe described earlier are only an example and the presentinvention is not limited to the example above.

However, it is preferred that the size n of a short TTI (the number ofsymbols) should be smaller than 7 to design a frame structure providinglow latency.

Sub-Band and Carrier/Cell-Wise 2-Level Frame Structure

Meanwhile, the sub-band and carrier/cell-wise 2-level frame structuredescribed earlier can be applied together. This will be described withreference to the drawings below.

FIG. 18 illustrates a radio frame structure according to one embodimentof the present invention.

With reference to FIG. 18, the band (or carrier/cell) 1 is set upaccording to a legacy frame structure, the band (or carrier/cell) 2 isset up according to the sub-band wise short TTI frame structure of thepresent invention, and the band 1 and 2 are carrier-aggregated withrespect to a particular UE.

Since the band 1 is the same as the example of FIG. 15, descriptionsrelated thereto will be omitted.

The band 2 is subdivided into a plurality of sub-bands (or sub-bandgroups), and more than one sub-band (or sub-band groups) among them canbe set up according to a short TTI frame structure.

More specifically, the PDCCH 1811 is allocated to a leading symbol foreach subframe and is allocated to a maximum of four leading symbols. Inother words, the PDCCH 1811 can be transmitted across the whole bandthrough a maximum of #0 to #3 symbol of each subframe. FIG. 18 assumesthat the PDCCH 1811 is allocated across the whole band through #0 and #1symbol of each subframe.

In what follows, for the convenience of descriptions, it is assumed thatthe PDCCH 1811 is mapped to the first two symbols for each subframe.

In the frequency region except for a sub-band dedicated to low latency,a PDSCH 1812 for normal data transmission can be allocated to theremaining symbols to which the PDCCH 1811 is not mapped. FIG. 18illustrate a case where a PDSCH 1812 is allocated across the wholefrequency region except for a sub-band dedicated to low latency through#3 to #13 symbol.

And more than one sub-band (or a sub-band group) for low latency canemploy a short TTI frame structure. More specifically, in the case of alegacy subframe, more than one sub-band (or sub-band group) subdividesthe symbols excluding those symbols to which the PDCCH 1811 has beenmapped (namely, the whole symbols of the legacy subframe except for thesymbol to which the PDCCH 1811 has been allocated) into n symbols ofwhich the size corresponds to the size of a short TTI (for example, 2 to4 symbol) and is composed of short TTI subframes (sPDSCHs 1813 andsPDSCHs 1814).

In other words, for each short TTI subframe, the sPDCCH 1813 isallocated to preceding, predetermined symbols (for example, one or twosymbols), and the sPDSCH 1814 is allocated to the remaining symbols.

Although the number of symbols for the sPDCCH 1813 is not limited, it ispreferred that the sPDCCH 1813 is composed of one symbol in case a shortTTI subframe consists of three symbols.

As shown in FIG. 18, in case the PDCCH 1811 is allocated to thepreceding two symbols of a legacy subframe, each subframe can comprisefour (=12/3) short TTI subframes (sPDCCH 1813 and sPDSCH 1814). And foreach short TTI subframe, the sPDCCH 1813 is allocated to the precedingone symbol, and the sPDSCH 1814 can be allocated to the remaining twosymbols.

In other words, a short TTI radio frame structure comprises four shortTTIs within one legacy subframe (namely, 1 ms), and one short TTIrepresents a data channel having a length of 3 symbols, which is about alength of 0.2 ms.

As described above, in case a subframe employs a short TTI framestructure, a short resource block (RB) can also be newly defined as aresource allocation unit for a low latency UE. For example, a short RBcan be defined such that it consists of 12 sub-carriers in the frequencydomain in the same way as in the prior art but consists of n symbols(namely, symbols having a short TTI size) in the time domain. Also, theshort RB may consist of x sub-carriers (x<12), of which the total numberis smaller than in the prior art, even in the frequency domain.

Also, in the same manner as in the legacy PDSCH, mapping of data ontoresource elements in the sPDSCH region can be first carried out in theincreasing order of frequency index and then in the increasing order ofsymbol index.

As shown in FIG. 18, in case a few sub-bands of a particular band employa short TTI frame structure, the control information (for example,frequency/time resource allocation information with respect to thedownlink data of the sPDSCH; MCS, NDI, RV, TPC command; and so on) withrespect to the downlink data transmitted from the sPDSCH 1814 can betransmitted through the corresponding sPDSCH 1814 and the sPDCCH beingmapped 1813 (namely, the sPDCCH transmitted most recently before thecorresponding sPDSCH). In other words, the control information withrespect to the downlink data transmitted from the first leftmost sPDSCH1814 is transmitted from the first sPDCCH 1813, and the controlinformation with respect to the downlink data transmitted from thesecond sPDSCH 1814 is transmitted from the second sPDCCH 1813, and thisscheme applies in the same way for the other cases.

Although the format of the sPDCCH 1813 employing a short TTI framestructure can be the same as that of the existing PDCCH 1811, it can bedefined by a different format. For example, in the sPDCCH 1813 format,one CCE can consist of x (x<9) REGs, and the REG mapped to the sPDCCH1813 region can consist of y (y<4) REs.

As described above, the band 1 employing a legacy frame structure or theband 2 of which a few sub-bands employing a short TTI frame structurecan operate as a PCell. Also, for a UE requiring low latency, by forcingthe band 1 employing a legacy frame structure and the band 2 of which afew sub-bands employing a short TTI frame structure to be aggregated,both of the band 1 and band 2 may be configured to operate as a PCell.

The symbols (or the number of symbols) to which the PDCCH 1811, sPDCCH1813, and sPDSCH 1814 are mapped; and the number of short TTIs employedwithin one legacy subframe described earlier are only an example and thepresent invention is not limited to the example above.

Method for Transmitting Configuration Information about a Short TTIFrame Structure

The radio resource information with respect to a short TTI framestructure according to the present invention can be transmitted throughan RRC message intended for transmitting cell information.

FIG. 19 illustrates a method for transmitting information about a shortTTI frame structure according to one embodiment of the presentinvention.

With reference to FIG. 19, the eNB transmits radio resource informationwith respect to a short TTI frame structure (in what follows, ‘short TTIradio resource information’) to the UE through an RRC message 51901.

At this time, an RRC message can correspond to a system informationmessage, an RRC connection setup message, an RC connectionreconfiguration message, or an RRC connection reestablishment message.

In what follows, specifics of short TTI radio resource information willbe provided.

1) The short TTI radio resource information can be transmitted to the UEthrough a cell-specific RRC message.

For example, the short TTI radio resource information can be transmittedbeing included in the ‘RadioResourceConfigCommon’ information element(IE) used for specifying radio resource configuration common to systeminformation or mobility control information.

The ‘RadioResouceConfigCommon’ IE can be transmitted being included in a‘MobilityControlInfo’ IE or a system information block type 2 (SIB-2)(or a newly defined SIB-x). The ‘MobilityControlInfo’ IE is an IE whichincludes a parameter related to a network controlled mobility controlledby a network within an E-UTRA.

The ‘MobilityControlInfo’ IE can be transmitted through an RRCconnection reconfiguration message. The RRC connection reconfigurationmessage is a command message for modifying an RRC connection.

Also, the ‘MobilityControlInfo’ can be transmitted through the SIB-2system information message. A system information message is used totransmit more than one system information block (SIB).

2) Since the short TTI radio resource information is used for a lowlatency UE, it may be transmitted to a low latency UE through aUE-specific RRC message.

For example, the short TTI radio resource information can be transmittedbeing included in a ‘pdschConfigDedicated’ IE or a‘physicalConfigDedicated’ IE used to specify UE-specific physicalchannel configuration.

The ‘pdschConfigDedicated’ IE or the ‘physicalConfigDedicated’ IE can betransmitted being included in a ‘RadioResourceConfigDedicated’ IE. The‘RadioResourceConfigDedicated’ IE is used to set up, modify, or releasea radio bearer (RB), to modify MAC main configuration, to modifyconfiguration of semi-persistent scheduling (SPS), and to modifydedicated physical configuration.

The ‘RadioResourceConfigDedicated’ IE can be transmitted being includedin the RRC connection setup message, RRC connection reconfigurationmessage, or RRC connection reestablishment message.

The RRC connection setup message is used to establish a signaling radiobearer (SRB), and the RRC connection reestablishment message is used tore-establish the SRB.

In what follows, information included in the short TTI radio resourceinformation will be described.

Frequency Resource Information for a Short TTI Subframe

This denotes frequency information about a sub-band to which a short TTIis applied within a frequency band. In case a sub-band and/orcarrier-wise 2-level radio frame structure is used, the information canbe included in short TTI radio resource information.

The frequency resource information for a short TTI subframe can beexpressed in units of a sub-carrier or an RB. For example, theinformation can be expressed in terms of index information about a startand/or end resource (namely, a sub-carrier or an RB). Also, theinformation can be expressed by the index about a start or end resource(namely, a sub-carrier or an RB) and the number of resources (namely, asub-carrier or an RB).

In the case of the sub-band wise 2-level frame structure (FIGS. 12 to14, FIG. 18), the frequency resource information for a short TTIsubframe represents frequency resource information about a particularsub-band employing a short TTI frame structure within a particular band.

On the other hand, in case a short TTI is applied across the whole of aparticular band (FIGS. 15 to 17), the information can be expressed interms of the highest/lowest resource (namely, a sub-carrier or an RB)index or set up by a predetermined value (for example, ‘0’).

The Number of Symbols for a Short TTI Subframe

This number represents the number of symbols with respect to one shortTTI subframe. For example, the short TTI subframe can be set to ‘3’consisting of three symbols.

In case a sub-band wise and/or carrier-wise 2-level radio framestructure is used, the number can be included in the short TTI radioresource information.

The Number of Symbols for an sPDCCH

This number represents the number of symbols about an sPDCCH within eachshort TTI subframe.

If an sPDCCH is present within a short TTI subframe (FIG. 13, FIGS. 17and 18), the number can be included in the short TTI resourceinformation.

The Number of Special Symbols

This number represents the number of special symbols found in a legacysubframe (namely, 1 ms). For example, the number can express two typesof information such as ‘1’ or ‘2’.

If a special symbol is found within a short TTI subframe (FIGS. 15 and16), the number can be included in the short TTI radio resourceinformation.

A UE requiring low latency can check radio resource information about ashort TTI in the corresponding band by receiving short TTI radioresource information transmitted through an RRC message as illustratedpreviously in FIG. 19 and transmit and receive data by using a short TTIstructure.

Meanwhile, the carrier/cell-wise 2-level frame structure describedearlier can be set up for a UE employing carrier aggregation.

At this time, a low latency UE needs to configure a PCell and an SCelldifferently from legacy UEs, which will be described below withreference to related drawings.

FIG. 20 illustrates a method for configuring carrier aggregation for acarrier-wise 2-level frame structure according to one embodiment of thepresent invention.

With reference to FIG. 20, the eNB transmits radio resource informationabout a short TTI frame structure (in what follows, it is called ‘shortTTI radio resource information’) to the UE through an RRC message 52001.

Since a method for transmitting and receiving short TTI radio resourceinformation and information included therein are the same as in thedescriptions of FIG. 19, descriptions thereof will be omitted.

The eNB transmits an RRC connection reconfiguration message to the UE toconfigure carrier aggregation S2002.

In case the UE carries out an attach process to a network through acarrier/cell employing a short TTI frame structure, the eNB can add acell having a legacy frame structure to the UE as an SCell through theRRC connection reconfiguration message. In other words, a carrier/cellhaving a legacy frame structure can be set up as an SCell.

The RRC connection reconfiguration message includes a‘radioResourceConfigDedicatedSCell’ field.

The ‘radioReourceConfigDedicatedSCell’ field includes a ‘SCellToAddMod’field. The ‘SCellToAddMod’ field is used for adding an SCell.

The ‘SCellToAddMod’ field includes a ‘sCellIndex’ field and a‘cellIdentification’ field.

And the ‘cellIdentification’ field includes a ‘physCellId’ field and a‘dl-CarrierFreq’ field.

The ‘sCellIndex’ field plays the role of a short identifier foridentifying an SCell and can include the index of a carrier/cellemploying a legacy frame structure.

The ‘physCellId’ field is used as a physical layer identifier of a celland can include a physical layer identifier of a carrier/cell employinga legacy frame structure. And the ‘dl-CarrierFreq’ field specifiesfrequency information of a cell and can include frequency information ofa carrier/cell employing a legacy frame structure.

FIG. 21 illustrates a method for configuring carrier aggregation forcarrier-wise 2-level frame structure.

With reference to FIG. 21, the eNB transmits radio resource informationabout a short TTI frame structure (in what follows, it is called ‘shortTTI radio resource information’) to the UE through an RRC message S2101.

Since the method for transmitting and receiving short TTI radio resourceinformation and the information included therein are the same as in thedescriptions of FIG. 19, descriptions thereof will be omitted.

The eNB transmits an RRC connection reconfiguration message to the UE toconfigure carrier aggregation S2102.

In case the UE carries out an attach process to a network through acarrier/cell employing a legacy frame structure, the eNB can add a cellhaving a short TTI frame structure to the UE as a secondary PCell(sPCell) through the RRC connection reconfiguration message. In otherwords, both of the carrier/cell having a legacy frame structure and thecarrier/cell having a short TTI frame structure can be set up as aPCell.

In this way, in case two carriers/cells are set up as a PCell, it ispreferable that control of each carrier/cell should be carried outdepending on service characteristics. In other words, a carrier/cellhaving a short TTI frame structure can be made to carry out only thecontrol dedicated for a service requiring low latency.

Having two PCells indicates that both of the two carriers/cells aremonitored (namely, a pDCCH, an sPDCCH, or a paging interval ismonitored) even if the UE enters an IDLE state or a dormant state.Therefore, any one of the two PCells may be defined as an SCell whichprovides the same meaning or carries out the same operation.

The RRC connection reconfiguration message includes a‘radioResourceConfigDedicatedSCell’ field.

The ‘radioResourceConfigDedicatedSCell’ field includes a‘sPCellToAddMod’ field. The ‘sPCellToAddMod’ field is used to add asecondary PCell.

The ‘sPCellToAddMod’ field includes a ‘sCellIndex’ field and a‘cellIdentification’ field.

And the ‘cellIdentification’ field includes a ‘physCellId’ field and a‘dl-CarrierFreq’ field.

The sPCellIndex′ field plays the role of a short identifier foridentifying an SCell and can include the index of a carrier/cellemploying a short TTI frame structure.

The ‘physCellId’ field can include a physical layer identifier of acarrier/cell employing a short TTI frame structure, and the‘dl-CarrierFreq’ field can include frequency information of acarrier/cell employing a short TTI frame structure.

Meanwhile, when adding an SCell or an SPCell to the UE as in FIGS. 20and 21, the eNB can transmit information about a cell employing a shortTTI radio frame structure to the UE.

In other words, the short TTI radio resource information described inthe example of FIG. 19 can be transmitted to the UE at the S2002 step ofFIG. 20 or at the S2102 step of FIG. 21. In this case, the S2001 step ofFIG. 20 or the S2101 step of FIG. 21 can be skipped.

FIG. 22 illustrates a method for transmitting and receiving dataaccording to one embodiment of the present invention.

With reference to FIG. 22, the eNB maps first downlink data to the PDSCHregion employing a radio frame structure based on a first TTI S2201.

As described in FIGS. 12 to 18, the eNB maps normal downlink data notneeding low latency to the PDSCH empoying a legacy radio framestructure. At this time, the first TTI can correspond to the existinglegacy TTI (namely, 1 ms).

The eNB maps second downlink data to the sPDSCH region employing a radioframe structure based on a second TTI S2202.

As described in FIGS. 12 to 18, the eNB maps downlink data requiring lowlatency to the PDSCH employing a short TTI radio frame structureaccording to the present invention. At this time, the second TTI canhave the same number of symbols in the sPDSCH region or the same numberof symbols in the sPDCCH and the sPDSCH region according to the presentinvention.

As described above, a subband-wise 2-level radio frame structure can beemployed for one frequency band, but a carrier-wise 2-level radio framestructure may be employed for another frequency band.

In case a carrier-wise 2-level radio frame structure is employed foranother frequency band, a cell employing a short TTI radio framestructure can be set up as a PCell. Also, both of a cell employing alegacy radio frame structure and a cell employing a short TTI radioframe structure may be set up as a PCell.

Both of the control information about first downlink data mapped to aPDSCH region and the control information about second downlink datamapped to an sPDSCH region can be transmitted to the PDSCCH employing alegacy radio frame structure.

Also, although the control information about first downlink data mappedto the PDSCH region are transmitted through a PDCCH employing a legacyradio frame structure, the control information about second downlinkdata mapped to the sPDSCH region can be transmitted through an sPDCCHemploying a short TTI radio frame structure according to the presentinvention.

Since description about downlink control information transmitted througha PDCCH or an sPDCCH are the same as in the descriptions of FIGS. 12 to18, descriptions thereof will be omitted.

Before transmitting the first downlink data and the second downlinkdata, the eNB can transmit short TTI radio resource information througha cell-specific RRC message or a UE-specific RRC message as illustratedin FIG. 19.

Meanwhile, although FIG. 22 assumed for the convenience of descriptionsthat the S2201 step precedes the S2202 step, the S2202 step can becarried out before the S2201 step.

Also, the S2201 and the S2202 step may be carried out simultaneously inthe time domain.

FIG. 23 illustrates a method for transmitting downlink data according toone embodiment of the present invention.

With reference to FIG. 23, the UE receives first downlink data in aPDSCH region employing a radio frame structure based on a first TTIS2301.

By blind-decoding the PDSCCH region, the UE obtains downlink controlinformation transmitted to the UE. And based on the obtained downlinkcontrol information, the UE decodes downlink data transmitted to the UEin the PDSCH region.

The UE receives second downlink data in an sPDSCH region employing aradio frame structure based on a second TTI S2302.

By blind-decoding the PDSCCH region or the sPDCCH region, the UE obtainsdownlink control information transmitted to the UE. And based on theobtained downlink control information, the UE decodes downlink datatransmitted to the UE in the sPDCCH region.

FIG. 23 illustrates a method for receiving downlink data through asubband-wise 2-level radio frame structure in a UE-specific manner orthrough a carrier-wise 2-level radio frame structure for differentfrequency bands.

The UE can receive short TTI radio resource information as illustratedin FIG. 19 from the eNB before carrying out the S2301 and the S2302step. Also, when the eNB configures carrier aggregation at the S2002step of FIG. 20 or at the S2102 step of FIG. 21 (namely, when an SCellor an sPCell is added), the UE can receive short TTI radio resourceinformation from the eNB.

Meanwhile, the present invention can be applied in the same way for anuplink band.

For example, according to the present invention, one legacy subframeincorporates 12/n (in the case of a normal CP) short subframes (aninteger value, 4 short subframes in case n=3) and 12$n special symbols(two special symbols in case n=3) through an TTI comprising n symbols(for example, three symbols). At this time, the 14% n special symbolscan be designed so that they can be used as contention-based resourcescapable of transmitting uplink data through contention among UEs withoutuplink resource allocation from the eNB.

The 2-level frame structure according to the present invention cantransmit and receive low latency data by using a new frame structurewhich minimizes an effect on legacy UEs.

More specifically, a legacy UE operating in a frequency band accordingto the present invention blind-decodes a PDCCH across the wholefrequency band of each subframe to receive data. And in case there iscontrol information being transmitted to the legacy UE, it receives thecorresponding data based on the information received through the PDCCH.

Since the eNB does not use a sub-band allocated for low latencyoperation for data scheduling of a legacy UE, it does not require thelegacy UE to carry out some new operation. Moreover, in case no data aretransmitted to a sub-band dedicated to low latency operation in thecorresponding 1 ms subframe, the eNB can use the same resourceallocation method as used for data transmission of a legacy UE forallocation of sub-band resources for low latency operation. Since theeNB controls PDSCH resource allocation with respect to a PDSCH, thepresent invention provides an advantageous effect that a legacy methodcan still be employed for legacy UEs.

At this time, a 5G UE requiring low latency can receive data much fasterby using a short TTI as it receive data in the sPDSCH resource region.However, although a data reception delay may incur a somewhat differenteffect according to a PDCCH mapping method with respect to an sPDSCH inthe various embodiments above, data can be received always with ashorter delay than the delay at the time of data transmission employingthe existing 1 ms subframe structure.

FIG. 24 illustrates a delay in radio transmission and receptionemploying a short TTI radio frame structure according to one embodimentof the present invention.

FIG. 24 illustrates a radio transmission and reception delay in view ofimplementation of downlink transmission and reception when 1 TTI is setup to comprise three symbols (namely, 0.213 ms).

With reference to FIG. 24, a propagation delay is generated between thetime the eNB starts transmission of downlink data and the time the UEstarts receiving the downlink data. And as the UE buffers the downlinkdata before decoding the downlink data, a buffering delay is generated.A delay due to buffering in the UE can amount to a total of about 0.071ms. A processing delay due to downlink data (and control information)decoding in the UE can amount to less than about 0.525 ms.

In this way, one-way over-to-air (OTA) latency from the eNB to the UEcan amount to less than about 0.6 ms.

And a delay due to preparation for ACK/NACK (for example, ACK/NACKencoding) and a propagation delay (PD) generated at the time of theACK/NACK consumes a total of less than about 0.3 ms for the UE.

In this way, for one-way data transmission, a transmitter-side (forexample, the eNB) may need about 1 ms of roundtrip OTA latency toreceive the ACK/NACK from a receiver side (for example, the UE).

After all, by using a short TTI frame structure of the presentinvention, the roundtrip OTA latency can be reduced by an amount ofabout 3 ms compared with the example of FIG. 11.

Apparatus for Implementing the Present Invention

FIG. 25 is a block diagram of a wireless communication apparatusaccording to an embodiment of the present invention.

Referring to FIG. 25, a wireless communication system includes an eNB2510 and a plurality of UEs 2520 belonging to the eNB 2510.

The eNB 2510 includes a processor 2511, a memory 2512, a radio frequency(RF) unit 2513. The processor 2511 may be configured to implement thefunctions, procedures and/or methods proposed by the present inventionas described in FIGS. 1-24. Layers of a wireless interface protocol maybe implemented by the processor 2511. The memory 2512 is connected tothe processor 2511 and stores various types of information for operatingthe processor 2511. The RF unit 2513 is connected to the processor 2511,transmits and/or receives an RF signal.

The UE 2520 includes a processor 2521, a memory 2522, and an RF unit2523. The processor 2521 may be configured to implement the functions,procedures and/or methods proposed by the present invention as describedin FIGS. 1-24. Layers of a wireless interface protocol may beimplemented by the processor 2521. The memory 2522 is connected to theprocessor 2511 and stores information related to operations of theprocessor 2522. The RF unit 2523 is connected to the processor 2511,transmits and/or receives an RF signal.

The memories 2512 and 2522 may be located inside or outside theprocessors 2511 and 2521 and may be connected to the processors 2511 and2521 through various well-known means. The eNB 2510 and/or UE 2520 mayinclude a single antenna or multiple antennas.

The aforementioned embodiments are achieved by combination of structuralelements and features of the present invention in a predeterminedmanner. Each of the structural elements or features should be consideredselectively unless specified separately. Each of the structural elementsor features may be carried out without being combined with otherstructural elements or features. Also, some structural elements and/orfeatures may be combined with one another to constitute the embodimentsof the present invention. The order of operations described in theembodiments of the present invention may be changed. Some structuralelements or features of one embodiment may be included in anotherembodiment, or may be replaced with corresponding structural elements orfeatures of another embodiment. Moreover, it will be apparent that someclaims referring to specific claims may be combined with another claimsreferring to the other claims other than the specific claims toconstitute the embodiment or add new claims by means of amendment afterthe application is filed.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the inventions. Thus, itis intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

INDUSTRIAL APPLICABILITY

Although the method for transmitting and receiving data in the wirelesscommunication system of the present invention is described mainly forthe example applied to 3GPP LTE/LTE-A system, it is also possible to beapplied to various wireless communication system as well as 3GPPLTE/LTE-A system.

1. A method for transmitting downlink data in a wireless communication system, comprising: mapping, by a eNB, first downlink data into a physical downlink shared channel (PDSCH) region according to a radio frame structure based on a first transmission time interval (TTI); mapping, by the eNB, second downlink data to a short PDSCH (sPDSCH) region according to a radio frame structure based on a second TTI; and transmitting, by the eNB, the first and the second downlink data.
 2. The method of claim 1, wherein a downlink cell into which the first downlink data are mapped is the same as a downlink cell into which the second downlink data are mapped, and one or more sub-bands within the downlink cell are formed according to a radio frame structure based on the second TTI.
 3. The method of claim 1, wherein a first downlink cell into which the first downlink data are mapped is different from a second downlink cell into which the second downlink data are mapped, and the first downlink cell and the second downlink cell are aggregated.
 4. The method of claim 3, wherein one or more sub-bands within the second downlink cell are formed according to a radio frame structure based on the second TTI.
 5. The method of claim 1, wherein size of the second TTI is the same as the number of symbols of the sPDSCH region.
 6. The method of claim 1, wherein size of the second TTI is the same as the sum of the number of symbols of a short physical downlink control channel (sPDCCH) region and the number of symbols of the sPDSCH region.
 7. The method of claim 3, wherein the first downlink cell is configured as a secondary cell, and the second downlink cell is configured as a primary cell.
 8. The method of claim 3, wherein both of the first downlink cell and the second downlink cell are configured as primary cells.
 9. The method of claim 1, further comprising transmitting, by the eNB, information about a radio frame structure based on the second TTI through a radio resource control (RRC) message.
 10. The method of claim 11, wherein the RRC message corresponds to one of a system information message, an RRC connection setup message, an RRC connection reconfiguration message, or an RRC connection reestablishment message.
 11. (canceled)
 12. A method for receiving downlink data in a wireless communication system, comprising: receiving, by an User Equipment (UE) first downlink data in a PDSCH region according to a radio frame structure based on a first TTI; and receiving, by the UE, second downlink data in an sPDSCH region according to a radio frame structure based on a second TTI.
 13. (canceled) 